Phobos, the larger of the two moons of Mars, is living on borrowed time. In 70 million years or so, it will move so close to Mars that it’ll either spiral into Mars or be pulverized by its gravity. If it’s pulverized, some of the debris may eventually coalesce to form a new moon.

In fact, a recent study says that could be how Phobos was born — from the remnants of an earlier moon.

For a while, the leading idea has been that Phobos and the other moon, Deimos, were born when a small planet crashed into the young Mars. The impact blasted out a huge amount of debris, which formed a ring around Mars. Some of this material coalesced to make Phobos and Deimos.

That scenario leaves a lot of questions about the little moons unanswered. But a new study says the big impact was only the beginning. Deimos may have been born from the debris of the impact, but Phobos was not. Instead, a much larger moon was. Over time, that moon dropped too close to Mars and was pulverized. Most of the debris fell onto Mars. But some of it formed another moon. That process may have played out several more times, dropping great piles of debris onto Mars while giving birth to ever smaller moons — all the way down to Phobos, which may continue the cycle.

Look for Mars near our own moon at dawn tomorrow. Mars looks like an orange star close to the upper right of the Moon. The true star Spica stands to the lower right of the Moon.